Thickness Shear Mode Resonator (TSM) or quartz crystal microbalance (QCM), is the oldest and simplest acoustic wave device, with good durability in harsh environments. A TSM usually comprises a thin disk of AT-cut quartz with parallel circular electrodes patterned on both sides. Running a voltage between the two electrodes results in a shear deformation of the crystal, causing the crystal to resonate as electromechanical standing waves are created. Wave displacement is maximized at the crystal faces, making the device sensitive to surface interactions. Typical TSM resonators operate between 5 and 30 MHz.
Surface Acoustic Wave (SAW) devices use waves with a longitudinal and a vertical shear component with virtually all their acoustic energy confined within one wavelength of a substrate's surface. These components can couple with a medium in contact with the substrate, as seen in FIG. 1, which strongly affects the amplitude and velocity of the wave and allows SAW sensors to directly sense mass and mechanical properties. SAW velocities are approximately 5 orders of magnitude less than the corresponding electromagnetic (bulk) wave, making SAWs among the slowest to propagate in solids. Typical SAW sensors operate from 25 to 500 MHz.
As it is known in the art, SAW devices are employed in a variety of applications, such as resonators and delay lines for oscillator circuits, filters and pressure transducers. Generally, a SAW device comprises at least one transducer including a set of conductive members which is disposed on or recessed within a surface of a piezoelectric substrate. One disadvantage of these devices is that Rayleigh mode SAWs have surface-normal wave components, seen in FIG. 2, making them poorly suited for liquid sensing, as the longitudinal waves caused by contact with a liquid excessively attenuate the surface wave.
In many applications of SAW devices, particularly with respect to applications of resonators and delay lines, as frequency stabilizing and determining elements in oscillators, it is important to provide a package having a relatively small size while, at the same time, properly mounting the SAW device within the package to reduce the so-called vibration sensitivity of the SAW device. SAW devices were originally used for electronic filtering of high frequency waves. Due to the nature of the waves, SAW devices became important as sensors, and may be used for biosensors and chemical sensors, and communication surface acoustic wave filters. SAW sensors may effectively detect trace amounts of chemical in a solution. A class of shear horizontal waves, Love-waves, are propagated in layered devices that concentrate the wave energy in a highly confined region near the surface. However, the sensors are limited by power consumption and reproducibility. As such, new SAW devices are needed for applications such as improvised explosive device discovery to diagnostic personal health care.